The present invention relates to a process for decoloring polyisocyanates containing isocyanurate and uretdione groups.
The isocyanate components employed in high-quality one- and two-component polyurethane paints having high light and weathering resistance are, in particular, polyisocyanate mixtures containing isocyanurate and uretdione groups.
These products are preferably prepared by catalytic oligomerization of aliphatic and/or cycloaliphatic diisocyanates, eg. 1-isocyanato-3,3,5-trimethyl-5-isocyanatomethylcyclohexane (IPDI) or 1,6-diisocyanatohexane (HDI).
Examples of catalysts which can be employed are hydroxides or organic salts of weak acids with tetraalkylammonium groups, hydroxides or organic salts of weak acids with hydroxyalkylammonium groups, or alkali metal salts or tin, zinc or lead salts of alkylcarboxylic acids.
The aliphatic and/or cycloaliphatic diisocyanates are reacted in the presence of a catalyst, if desired with use of solvents and/or auxiliaries, until the desired conversion has been achieved. The reaction is then terminated by deactivating the catalyst, and the excess monomeric diisocyanate is removed by distillation. Depending on the catalyst type and reaction temperature used, polyisocyanates containing various proportions of isocyanurate and uretdione groups are obtained.
The products prepared in this way are mostly clear, but have a certain yellow coloration depending on the catalyst type, the diisocyanate quality, the reaction temperature and the reaction procedure.
However, products having a low color index are desired for the preparation of high-quality polyurethane paints. A number of processes are known from the prior art for reducing the color index of such products. Thus, DE-A-38 06 276 proposes reducing the carbon dioxide content of the HDI employed as monomer to less than 20 ppm before the oligomerization by degassing under reduced pressure and subsequently blowing nitrogen through the HDI, and employing quaternary ammonium hydroxides as the oligomerization catalyst. However, the carbon dioxide removal step is technically very complex.
EP-A-0 339 396 proposes using quaternary ammonium fluorides as the trimerization catalyst. Although this process tolerates a higher carbon dioxide content, the catalyst employed must, however, be chemically deactivated. The resultant compounds remain in the product and may result in applicational problems on further processing. A further way of preparing low-color-index polyisocyanates containing isocyanurate groups is the addition, proposed in EP-A-0 336 205, of polyester-diols to the starting diisocyanate. This allows the amount of catalyst employed to be reduced, but the resultant products are still relatively highly colored.
According to EP-A-0 377 177, aliphatic diisocyanates are oligomerized in the presence of phosphines as catalyst, and, after termination of the oligomerization, some of the unreacted diisocyanate is removed by distillation and some is converted into urethane by addition of alcohol. The reaction product is subsequently treated with peroxides. Although the peroxide treatment significantly reduces the color index of the oligomerization product, the use of peroxides is, however, associated with problems. Thus, peroxides are frequently difficult to handle industrially. Peroxides which are safer to handle are usually supplied in solution, but the dibutyl phthalate frequently used as solvent causes applicational problems in the preparation of paints.
It is an object of the present invention to provide a simple process for the effective decoloration of polyisocyanates containing isocyanurate and uretdione groups which avoids the disadvantages of the prior art.
We have found that, surprisingly, this object is achieved by treating the demonomerized polyisocyanates containing isocyanurate and uretdione groups with oxygen.